The present invention relates to a novel pixel structure that employs at least one organic thin-film transistor (OTFT). More specifically, the present invention provides a new transistor structure where an OTFT electrode on the data-line side of the transistor is extended or wrapped around three sides of the transistor relative to a pixel-side electrode.
A conventional pixel is shown in FIG. 1. The source and drain contacts for the TFT are made from a metallization layer (such as palladium) that makes an ohmic contact to the organic layer. This metallization layer is also used to form the data lines (column lines) of the display, where it contacts one side of the pixel transistor. The transparent indium-tin oxide (ITO) pixel electrode contacts the metallization on the other side of the transistor. A xe2x80x9cpadxe2x80x9d of metallization contacts the TFT and lies over the ITO, as shown in FIG. 1. The gate metallization lies under a gate dielectric layer (such as silicon dioxide or silicon nitride) that separates it from the organic layer.
The structure of FIG. 1 has high capacitance between the gate electrode and the source-drain metallization that runs out to the ITO pixel electrode, where the parasitic capacitance is roughly proportional to the area of overlap between the two electrodes. This is a serious issue, since as the transistor makes the transition from an xe2x80x9conxe2x80x9d state to an xe2x80x9coffxe2x80x9d state, this capacitance results in a large capacitive xe2x80x9cpush-downxe2x80x9d (for n-channel TFTs) or xe2x80x9cpush-upxe2x80x9d (for p-channel TFTs) in the pixel voltage when the transistor is turned off. This change in the pixel voltage is about 15 volts for the pixels that are fabricated using the conventional geometry. The capacitance is non-uniform from pixel to pixel, so it leads to non-uniform pixel voltages across the display, i.e., the light intensity across the display will also vary and will be noticeable. It also requires the OTFTs to be able to withstand larger voltages between gate, source, and drain, than they would in the absence of the capacitive effect.
Therefore, a need exists for a novel pixel structure having an OTFT structure that minimizes gate-to-pixel capacitance.
The present invention is a novel pixel structure having at least one OTFT that minimizes gate-to-pixel capacitance. In one embodiment, the present invention provides a new transistor structure where an OTFT electrode on the data-line side of the transistor is extended or wrapped around three sides of the transistor relative to a pixel-side electrode. Thus, the pixel-side electrode can be made very small without reducing the effective width (and therefore on-current) of the transistor. Using this novel structure, it is possible to reduce the gate-to-pixel capacitance several-fold.